Conventional resistance spot welding techniques employ a method by which metal surfaces are joined together in one or more spots. Workpieces are held together under force by one or more electrodes. The contacting surfaces are heated by a pulse of high amperage current generated by contact with the electrodes to form what is known as a weld nugget at the interface between the two surfaces. When the flow of current ceases, the electrode force is ordinarily maintained for a short period of time to allow the weld nugget to cool and solidify forming a strong mechanical bond. An excellent discussion of the details of the metallurgical phenomena that occurs during resistant spot welding is found in Nied "The Finite Element Modelling of the Resistance Spot Welding Process", Welding Research Supplement, pp. 123-132 (Apr., 1984).
The popularity of resistance spot welding is due in large part to its capability of rapidly producing welds with apparatus that can be used in automated production For instance, U.S. Pat. No. 4,861,959 to Cecil discloses a resistance welding gun for cooperation with a robotic arm. This type of device affords a production system great versatility for high volume applications.
While resistance spot welding has many advantages, it is difficult to control the process satisfactorily in order to produce consistently good welds. Many different factors must be controlled such as voltage, current, pressure, heat loss, shunting, and electrode wear, as well as the thickness and composition of the workpiece material. Many of these variables are difficult to consistently control.
Several attempts have been made to automatically control resistance spot welding processes. For example, some techniques have been designed to regulate the amount of energy used during the weld cycle. To this end, current sensors and voltage regulators have been incorporated into feedback systems to compare the detected levels with certain preset references. These feedback systems are disadvantageous from the standpoint that they do not directly detect physical characteristics of the weld itself but instead rely upon detection of secondary parameters. This can lead to poor weld quality when uncontrolled parameters vary from nominal operating conditions.
Other techniques provide means for determining whether the metal of the workpieces have reached a molten state. If the metals to be welded do not reach the temperature required to become molten, an insufficient weld could result. It has been shown through measurements that when the molten state is reached, the electrodes, which are being forced against the workpiece, begin to move into the metal. Accordingly, it has been suggested that the detection of melting by sensing subsequent inward movement of the electrodes, called indentation or penetration, is a potentially good way of determining the state of the weld. However, just because the metal reaches a molten state, does not always ensure that a good weld is made. For example, too much weld current will produce melting, but will not necessarily produce the formation of the weld nugget which is an important factor in generating a good weld. Other parameters will effect the size and configuration of the weld nugget and the many prior techniques of merely sensing inward movement of the electrodes into the workpieces cannot readily determine the extent of weld nugget growth. Thus, penetration alone is insufficient to determine weld quality.
U.S. Pat. No. 4,419,558 to Stiebel accomplishes the detection of electrode travel indirectly by utilizing a load cell to monitor the squeezing force applied through the electrodes to the workpieces. Among the disadvantages with this construction is that its sensor is located very close to the position at which the weld is made and it does not lend itself to incorporation into many welding gun designs.
An improved resistance spot welding apparatus is disclosed by U.S. Pat. No. 4,542,277 to Cecil for automatically and consistently detecting the quality of resistance spot welds. This device includes a cylinder with two opposite ends and a piston assembly within the cylinder. This piston assembly has a piston rod extending through both ends of the cylinder. One end of the piston rod is coupled to an electrode for making the weld and the opposite end of the piston rod is coupled to a sensor assembly for detecting the quality of the resulting weld as a function of movement of the piston rod.
This device, while having many innovative features, requires a custom two-ended cylinder for mounting the sensor assembly. A problem with this configuration is that it is not suitable for welding applications which require the space to the back of the cylinder for other functions such as the placement of mounting assemblies. In this case a two-ended cylinder would not be feasible. Further, the two-ended cylinder configuration does not provide for electrical isolation of the sensor necessary in welding operations due to the high currents produced. Lack of electrical isolation produces noise on the sensor signal. Some of the noise can be eliminated using software processing algorithms or processing circuitry. However, this extra processing cannot be provided without cost, and at best, it will filter some of the desired signal and leave some of the noise behind.